Silicon wafer and method for producing the same

ABSTRACT

An n-type wafer is provided having a &lt;111&gt; crystal axis in which the resistivity distribution in the surface of the wafer is uniform. The wafer is suitable for use in, e.g., a zener diode. A method is provided for growing a single crystal of n-type silicon doped with a group V element such as phosphorus using the Czochralski method or the floating zone melting (FZ) method wherein the center axis of the silicon single crystal is tilted by a tilt angle of 1-6 degrees from the &lt;111&gt; crystal axis. The silicon single crystal is sliced obliquely at the angle corresponding to the tilt angle to yield an n-type wafer having a &lt;111&gt; crystal axis.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an n-type silicon wafer having a<111> crystal axis suitable for a zener diode and the like, and to amethod for producing such a silicon wafer.

[0003] 2. Discussion of the Background

[0004] A zener diode in general has a structure in which an n-typesilicon substrate is selectively doped with a p-type impurity. As then-type silicon substrate, a silicon wafer doped with a group V element,such as phosphorus, is used so that its specific resistance is as low asseveral milliohms to several ohms. As the silicon wafer, a wafer havinga <111> crystal axis is preferred from the viewpoint of electricalproperties, in particular its operating resistance, which is normallyproduced through a growing process by the Czochralski method.

[0005] Since the resistivity distribution in the surface of an n-typewafer having a <111> crystal axis is uneven, the use of n-type wafershaving a <100> crystal axis or a <511> crystal axis is also consideredin some fields (e.g. Japanese Patent Application Laid-Open No.4-266065).

[0006] However, the electrical properties of n-type wafers having<100>or <511> crystal axes are typically inferior to n-type wafershaving a <111> crystal axis. Specifically, n-type wafers having <100> or<511> crystal axes can be used only within a certain operating voltagesection. Due to this problem, n-type wafers having a <111> crystal axisare still desired as n-type wafers for zener diodes. However, n-typewafers having a <111> crystal axis have the following problems relatedto the properties of zener diodes.

[0007] One of the properties of zener diodes is that their operatingvoltage sections are divided into very narrow sections. When used as thematerial for zener diodes, therefore, silicon wafers are required tohave an even resistivity distribution in the wafer surface. However,n-type wafers having a <111> crystal axis have a problem that theevenness of the resistivity distribution is essentially poor. Therefore,the yield of good products in the wafers is considerably low.

[0008] In order to solve such a problem, it is effective to some extentto reduce the rotation rate of the crucible in the process for growing asingle crystal rod, which is the material for silicon wafers, to makethe doping element distribution even in the direction of the crystaldiameter. In fact, the rotation rate is considerably lower than therotation rate used in ordinary growing by the Czochralski method. Whenthe rotation rate of the crucible is reduced, however, the convection ofthe molten silicon tends to transport foreign substances toward thecrystal, accelerating the dislocation. Therefore, the yield of singlecrystals is lowered, and, from this point of view, the rotation ratecannot be greatly reduced.

[0009] The uneven distribution of the doping element in a single crystalis more significant as the crystal diameter increases. In order toprevent uneven distribution, further reduction of the rotation rate ofthe crucible is required, but this accelerates the dislocation.

[0010] For these reasons, the resistivity distribution in the surface ofan n-type wafer having a <111> crystal axis, as represented by Δρ={(ρmax−ρ min)/ρ min)}/×100, cannot be kept at 10% or less. In addition, thecrystal diameter is limited to 4 inches or less.

[0011] The above description is not limited to wafers produced by theCzochralski method; the same problems exist in wafers produced by thefloating zone melting (FZ) method.

[0012] In order to solve these problems in n-type wafers having a <111>crystal axis, a method is known to radiate neutrons in a nuclear reactorto convert a part of an isomer of silicon into phosphorus. However,without doping, this method is expensive. Thus, there still exists aneed for providing a practical and inexpensive n-type water having a<111> crystal axis having an even resistivity distribution in itssurface.

SUMMARY OF THE INVENTION

[0013] One object of the present invention is to provide an n-type waferdoped with a group V element, such as phosphorus, and having a <111>crystal axis and a uniform resistivity distribution in its surface.

[0014] In order to achieve the above and other objects of the presentinvention, the present inventors tilt the center axis of a singlecrystal slightly.

[0015] By tilting the center axis of a single crystal slightly from the<111> crystal axis, the distribution of the quantity of a doping elementin the radial direction of the crystal is made uniform. Wafers obliquelysliced from a single crystal at an angle corresponding to the tilt angle(the angle defined by the center axis of the single crystal and the<111> crystal axis) have a <111> crystal axis; since the tilt angle issmall, the loss of any material due to tilting is slight. Even withoutusing neutron irradiation, the resistivity distribution in the surfaceof the doped sliced wafers, represented by Δρ={(ρ max−ρ min)/ρ min}×100can be kept at 10% or less.

[0016] The first embodiment of the present invention relates to ann-type silicon wafer doped with a group V element, having a <111>crystal axis, and a resistivity distribution in the surface representedby Δρ={(ρ max−ρ min)/ρ min}× 100 of 10% or less.

[0017] The second embodiment of the present invention relates to amethod for producing a silicon wafer that includes growing a singlecrystal of n-type silicon doped with a group V element through the useof the Czochralski method or the floating zone melting (FZ) method sothat the center axis of the single crystal is tilted by 1-6 degrees fromthe <111> crystal axis, then slicing a wafer from the grown singlecrystal obliquely at an angle corresponding to the tilt angle so thatthe sliced wafer has a <111> crystal axis.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a diagram illustrating the embodiments and advantages ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0019] A more complete appreciation of the invention and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription of the preferred embodiments when considered in connectionwith the accompanying drawings, which are not intended to be limitingunless otherwise specified.

[0020] In a preferred embodiment of the present invention, as FIG. 1(a)shows, a silicon single crystal 3 is grown from molten silicon 2 dopedwith a predetermined level of a group V element, such as phosphorus,arsenic, nitrogen, and antimony in a crucible 1 by the Czochralskimethod. At this time, a seed crystal 4 the center axis of which istilted by 1-6 degrees (tilt angle θ) from the <111> crystal axis isused. The seed crystal is applied according to methods known in the art.Thus, the center axis of the grown silicon single crystal 3 is tilted by1-6 degrees from the <111> crystal axis. Due to such tilting, thequantitative distribution of the doping element in the single crystal ismade even in the radial direction of the crystal.

[0021] The reason why the quantitative distribution of the dopingelement in the single crystal is made even in the radial direction ofthe crystal by tilting the center axis of the crystal by 1-6 degreesfrom the <111> crystal axis is believed to be as follows:

[0022] Uneven distribution of the quantity of the doping element in theradial direction of the crystal in the silicon single crystal 3 isbelieved to result from the variation in the coefficient of segregationin the radial direction of the crystal due to the effects of therotation rate of the silicon single crystal 3 and the solidifying rateof molten silicon 2.

[0023] That is, the silicon single crystal 3 grown from molten silicon 2rotates around the axis relative to molten silicon 2, and, as a result,the circumferential speed of the silicon single crystal 3 is lower atthe central portion, and higher at the periphery of the crystal. Thequantity of the doping element that is incorporated in the siliconsingle crystal 3 from molten silicon 2 depends upon the coefficient ofsegregation, and the coefficient of segregation tends to increase as therelative circumferential speed at the solidifying interface of thesingle crystal decreases. Because of this tendency, the concentration ofthe doping element in the silicon single crystal 3 is higher at thecentral portion of the crystal. The result on the resistivitydistribution in the surface of the wafer is that the resistancedecreases toward the central portion of the crystal where theconcentration of the doping element is high, as FIG. 1(b) shows. Sincethis phenomenon is not affected by the direction of the crystal axis, itoccurs not only in wafers having a <111> crystal axis, but also inwafers having <100> or <511> crystal axes. Furthermore, it occurssimilarly when the center axis of the crystal is tilted by 1-6 degreesfrom the <111> crystal axis.

[0024] Whereas the effect of the solidifying rate of the molten silicon2 is controlled by the direction of the crystal axis, in the siliconsingle crystal 3 having a <111> crystal axis, since linkage betweenatoms in the lateral direction perpendicular to the center axis isstrong, the crystal growth rate in the lateral direction is high. Here,since the solidifying interface of the single crystal is normally anupwardly convex surface, the solidifying rate at the horizontal top areais higher than the solidifying rate on other areas. The higher thesolidifying rate, the less the doping element is discharged from thecrystal into the melt. The larger the coefficient of segregation is, thelarger the concentration of the doping element becomes in the siliconsingle crystal 3 having a <111> crystal axis in the area closer to thecentral portion of the crystal. The effect on the resistivitydistribution in the surface of the wafer is that the resistancedecreases toward the central portion of the crystal where theconcentration of the doping element is high, as FIG. 1(c) shows. Sincethis phenomenon is unique in a <111> crystal axis, the resistivity atthe central portion of the wafer having a <111> crystal axis is greatlyreduced due to the effect of the rotation of the single crystal, asdescribed above, and the effect of the solidifying rate, and theresistivity distribution in the surface becomes very uneven. This is thereason why the resistivity distribution is extremely uneven in aconventional wafer having a <111> crystal axis.

[0025] The reason why the resistivity distribution in the surface ofwafers having <100> or <511> crystal axes is even is that an extremedecrease in resistivity at the central portion, which is unique to the<111> crystal axis, does not occur in these wafers. However, these <100>or <511> wafers are not suitable as the material for zener diodes fromthe point of view of electrical properties as described above.

[0026] On the other hand, in the case of growing a silicon singlecrystal 3 according to the invention wherein the center axis is slightlytilted from the <111> crystal axis as shown in FIG. 1(a), the portion ofa high solidifying rate, that is, the portion of the high concentrationof the doping element, moves laterally from the central portion of thecrystal due to this tilting. Therefore, with respect to the resistivitydistribution in the surface of the wafer, the resistance decreases attwo locations at both sides of the central portion of the crystal asFIG. 1(d) shows, and the problem of resistance decrease at the centralportion has been solved.

[0027] Since, in a silicon single crystal 3 having a center portionslightly tilted from the <111> crystal axis, the effect shown in FIG.1(c) overlaps with the effect shown in FIG. 1(d) resulting in thedistribution of the doping element as shown by the solid line in FIG.1(e), and an increase in the doping element concentration in the centralportion of the crystal is lessened. Thus, the concentration distributionof the doping element in the radial direction of the crystal is madesignificantly even, when compared with the silicon single crystal havinga <111> crystal axis aligned with the center. This is why thequantitative distribution of the doping element in the radial directionof the crystal according to the invention becomes even by tilting thecenter axis of the crystal by 1-6 degrees from the <111> crystal axis.

[0028] When a silicon single crystal 3, the center axis of which isslightly tilted from the <111> crystal axis has been grown, is obliquelysliced at an angle corresponding to the tilt angle to form siliconwafers having a <111> crystal axis, according to the present invention,an even resistivity distribution in the surface corresponding to theconcentration distribution of the doping element in the silicon singlecrystal 3 is obtained. As a result, an n-type wafer having a <111>crystal axis of a Δρ of 10% or less, which cannot be achieved in thedoping type, is produced.

[0029] Moreover, since the crucible 1 does not need to rotate at anextremely low rate because of the even distribution of the dopingelement concentration in the silicon single crystal 3, the dislocationis inhibited, and the yield of the dislocation-free single crystal isimproved. Also, the growth of single crystals having as large a diameteras 5 inches or more becomes possible.

[0030] Preferably, the crucible rotation rate is greater than 0.3 rpm,more preferably more than 0.8 rpm, more particularly preferably morethan 2 rpm, more especially preferably more than 5 rpm, most preferablymore than 6-8 rpm, most especially preferably more than 9-10 rpm.

[0031] Preferably, the single crystal diameter is 4 inches or more. Morepreferably the single crystal diameter is 5 inches or more.

[0032] Furthermore, since the tilt angle of the center axis of thesilicon single crystal 3 from the <111> crystal axis is small, the tiltangle of slicing the crystal to form wafers is also small, and the lossof the material due to oblique slicing is prevented. Also, since theellipticity of the sliced wafer is slight, the loss of the material dueto ellipticity is minimized.

[0033] In the case of a <511> crystal axis for the reference, since itis tilted by 15 degrees or more from the <111> crystal axis, the loss ofthe material due to oblique slicing and due to ellipticity becomessignificant even if wafers having a <111> crystal axis are obtained.

[0034] The reason why the tilt angle is preferably limited to 1-6degrees in the method for producing wafers according to the presentinvention is that if the tilt angle is less than 1 degree, theresistivity distribution in the surface of the wafer cannot besufficiently even, and if the tilt angle exceeds 6 degrees, the loss ofthe material due to oblique slicing and due to ellipticity becomessignificant. An especially preferred tilt angle is a lower limit of 2degrees or more and an upper limit of 5 degrees or less. More especiallypreferred is a tilt angle between 3 and 4 degrees. These tilt angleranges include all values and subranges therebetween, including 1.2,5.9, 2.5, and 3.5.

EXAMPLES

[0035] Having generally described this invention, a furtherunderstanding can be obtained by reference to the following examples,which are provided for purposes of illustration, and are not intended tobe limiting unless otherwise specified.

[0036] Phosphorus doped 4-inch and 5-inch silicon single crystals forzenor diodes were grown under the conditions shown in Tables 1 and 2.The yield of dislocation-free crystals in crystal growth, and theresistivity ρ in the surface of wafers taken from the top, central, andbottom portions of the grown crystals are shown in Table 2.

[0037] The mean value of the resistivity ρ in the surface of waferstaken from the top, central, and bottom portions of the grown crystalsare shown in Table 2.

[0038] The mean value of the resistivity ρ lowers gradually from the topportion to the bottom portion with the change with the passage of timein the phosphorus concentration in the melt. The distribution of theresistivity ρ is represented by Δρ={(ρ max−ρ min)/ρ min}×100%. TABLE 1Crystal Diameter 4 inches 5 inches Crystal pulling rate 1.0 mm/min 0.9mm/min Tilt angle of center See Table 2 See Table 2 axis of crystalCrystal rotation rate 25 rpm 25 rpm Crucible rotation rate See Table 2See Table 2 Pressure in crucible 1330 pa 1330 pa Atmosphere in crucibleAr gas Ar gas Crystal pulling length 1000 mm 1000 mm

[0039] TABLE 2 Conventional Comparative Comparative Example Example 1Example 2 Example 1 Example 2 Example 3 Example 4 Crystal diameter 4inches 4 inches 5 inches 4 inches 4 inches 4 inches 5 inches Tilt angleof crystal center axis 0° 0° 0° 5° 5° 1° 5° Crucible rotation rate 0.8rpm 10 rpm 0.3 rpm 0.8 rpm 7 rpm 7 rpm 7 rpm Resistivity Top Mean 23.6*23.8* 23.5* 24.0* 23.2* 23.5* 23.3* in the Δρ 11% 23% 12% 6% 8% 4% 8%surface ρ Center Mean 16.5* 16.6* 16.0* 16.8* 16.6* 15.9 16.1 Δρ 15% 16%13% 3% 3% 6% 4% Bottom Mean 14.0* 14.5* 14.1* 14.7* 14.2* 14.0* 14.5* Δρ13% 27% 15% 2% 5% 9% 5% Yield of dislocation-free crystals 27% 83% 11%25%  75%  88%  70% 

[0040] In the Conventional Example, a 4-inch single crystal was grownaccording to conventional growing conditions. The center axis had a tiltangle from the <111> crystal axis of 0 degrees and coincided with thecrystal axis. The rotation rate of the crucible was 0.8 rpm. The grownsilicon single crystal was sliced in the direction perpendicular to thecentral axis to obtain n-type wafers having a <111> crystal axis.

[0041] The resistivity distribution in the surface of the thus obtainedConventional Example wafers was 11% at a minimum. The mean yield ofdislocation-free crystals in the growing process for 45 crystals was27%.

[0042] In Comparative Example 1, a 4-inch single crystal was grown. Thecenter axis had a tilt angle from the <111> crystal axis of 0 degreesand coincided with the crystal axis. The rotation rate of the cruciblewas higher than that of the Conventional Example (10 rpm) in order toelevate the yield of dislocation-free crystals. The grown silicon singlecrystal was sliced in the direction perpendicular to the central axis toobtain 4-inch n-type wafers having a <111> crystal axis.

[0043] Since the rotation rate of the crucible was raised, the meanyield of dislocation-free crystals in the growing process for 12crystals was improved to 83%. However, the resistivity distribution inthe surface of the thus obtained Comparative Example 1 wafers was muchmore uneven than in the Conventional Example, and was 16% at a minimum.

[0044] In Comparative Example 2, a 5-inch single crystal was grown. Thecenter axis had a tilt angle from the <111> crystal axis of 0 degreesand coincided with the crystal axis. The rotation rate of the cruciblewas lower than in the Conventional Example (0.3 rpm) because thediameter of the single crystal was increased. The grown silicon singlecrystal was sliced in the direction perpendicular to the central axis toobtain 5-inch n-type wafers having a <111> crystal axis.

[0045] The resistivity distribution in the surface of the thus obtainedComparative Example 2 wafers was 12% at a minimum, almost the same asthat of the Conventional Example, and still exceeded 10%. The mean yieldof dislocation-free crystals in the growing process for 9 crystals wasmarkedly lowered to 11% because of the low rotation rate of thecrucible. Due to this low yield, practical operation is impossible.

[0046] In Example 1, a 4-inch single crystal was grown according to theinvention. The center axis was tilted at an angle of 5 degrees from the<111> crystal axis. The rotation rate of the crucible was 0.8 rpm, thesame as in the Conventional Example. The grown silicon single crystalwas obliquely sliced at an angle of 5 degrees from the center axis toobtain 4-inch n-type wafers having a <111> crystal axis according to theinvention.

[0047] The mean yield of dislocation-free crystals in the growingprocess for 8 crystals was 25% because the rotation rate of the cruciblewas the same as that of the Conventional Example; however, theresistivity distribution in the surface of the thus obtained Example 1wafers was 6% at a maximum, which is significantly more even than theConventional Example.

[0048] In Example 2, a 4-inch single crystal was grown according to theinvention. The center axis was tilted at an angle of 5 degrees from the<111> crystal axis as in Example 1. The rotation rate of the cruciblewas higher than in the Conventional Example (7 rpm). The grown siliconsingle crystal was obliquely sliced at an angle of 5 degrees from thecentral axis to obtain 4-inch n-type wafers having a <111> crystal axisaccording to the invention.

[0049] The mean yield of dislocation-free crystals in the growingprocess for 8 crystals was improved to 75%, because the crucible wasrotated at a higher rate than in the Conventional Example. Theresistivity distribution in the surface of the thus obtained Example 2wafers was 8% at a maximum despite the high rotation rate of thecrucible, and the target value of 10% or less was achieved.

[0050] In Example 3, a 4-inch single crystal was grown according to theinvention. The center axis was tilted at an angle of 1 degree from the<111> crystal axis. The rotation rate of the crucible was 7 rpm as inExample 2. The grown silicon single crystal was obliquely sliced at anangle of 1 degree from the central axis to obtain 4-inch n-type wafershaving a <111> crystal axis according to the invention.

[0051] The mean yield of dislocation-free crystals in the growingprocess for 8 crystals was improved to 88%, because the crucible wasrotated at a higher rate than in the Conventional Example. Theresistivity distribution in the surface of the thus obtained Example 3wafers was 9% at a maximum despite the high rotation rate of thecrucible.

[0052] In Example 4, a 5-inch single crystal was grown according to theinvention. The center axis was tilted at an angle of 5 degrees from the<111> crystal axis as in Examples 1 and 2. The rotation rate of thecrucible was 7 rpm as in Examples 2 and 3. The grown silicon singlecrystal was obliquely sliced at an angle of 5 degrees from the centralaxis to obtain 5-inch n-type wafers having a <111> crystal axisaccording to the invention.

[0053] The resistivity distribution in the surface of the thus obtainedExample 4 wafers was 8% at a maximum despite the large diameter (a5-inch wafer), and the target value of 10% or less was achieved. Themean yield of dislocation-free crystals in the growing process for 10crystals was improved to 70% similar to Example 2. This is the yieldlevel which enables practical operation.

[0054] As described above, since the silicon wafer of the presentinvention is a doped type wafer doped with a group V element, havingexcellent uniformity of surface resistivity distribution and excellentyield, the manufacturing costs of zener diodes may be significantlyreduced. In addition, the method for producing wafers of the presentinvention contributes to the further reduction of the manufacturingcosts of zener diodes by inhibiting the loss during wafer slicing insilicon wafer production.

[0055] This application is based on Japanese Patent Application NumberHEI 9-367434, filed Dec. 24, 1997, the entire contents of which arehereby incorporated by reference.

[0056] Having now fully described the invention, it will be apparent toone of ordinary skill in the art to which this invention pertains thatmany changes and modifications may be made thereto without departingfrom the spirit or scope of the invention as set forth herein.

What is claimed as new and desired to be secured by letters patent inthe United States is:
 1. A silicon wafer, comprising: an n-type siliconwafer having a <111> crystal axis, doped with a group V element; whereina resistivity distribution in the surface of the wafer, represented byΔρ={(ρ max−ρ min)/ρ min}×100, is 10% or less.
 2. The silicon wafer ofclaim 1, wherein the resistivity distribution, Δρ is 5% or less.
 3. Thesilicon wafer of claim 1, wherein the resistivity distribution, Δρ is 2%or less.
 4. The silicon wafer of claim 1, wherein the silicon wafer hasa diameter of 4 inches or more.
 5. The silicon wafer of claim 1, whereinthe silicon wafer has a diameter of 5 inches or more.
 6. The siliconwafer of claim 1, wherein the surface of the silicon wafer comprises aplane perpendicular to the <111> crystal axis.
 7. The silicon of claim1, wherein said group V element is selected from the group consisting ofphosphorous, arsenic, antimony, and nitrogen.
 8. The silicon wafer ofclaim 1, wherein said group V element is phosphorous.
 9. The siliconwafer of claim 1, wherein said group V element is antimony.
 10. Thesilicon wafer of claim 1, wherein said group V element is arsenic.
 11. Amethod for producing a silicon wafer, comprising: growing a singlecrystal of n-type silicon doped with a group V element with a methodselected from the group consisting of the Czochralski method and thefloating zone melting (FZ) method to obtain a grown single crystal,wherein a center axis of the grown single crystal is tilted by a tiltangle of 1-6 degrees from the <111> crystal axis; and slicing the grownsingle crystal obliquely at an angle corresponding to the tilt angle toobtain a sliced wafer having a <111> crystal axis.
 12. The method ofclaim 11, wherein the tilt angle is between 2 and 5 degrees.
 13. Themethod of claim 11, wherein the tilt angle is between 3 and 4 degrees.14. The method of claim 11, further comprising growing the singlecrystal at a crucible rotation rate of more than 2 rpm.
 15. The methodof claim 11, further comprising growing the single crystal at a cruciblerotation rate of more than 10 rpm.
 16. A silicon wafer, prepared by theprocess of claim
 11. 17. A silicon wafer, prepared by the process ofclaim 11, having a resistivity distribution in the surface of the wafer,represented by Δρ={(ρ max−ρ min)/ρ min}×100, of 10% or less.